Method of forming a coated body having a nanocrystalline CVD coating of Ti(C,N,O)

ABSTRACT

A method of forming a coated body having a nanocrystalline CVD coating of Ti(C,N,O) is disclosed. The coating is formed using the MTCVD process and including, as part of the gaseous mixture, CO, CO 2  or mixtures therof. The use of this dopant during the coating results in a much smaller, equiaxed grain size.

This application is a divisional of application Ser. No. 09/487,495,filed on Jan. 19, 2000 now U.S. Pat. No. 6,472,060.

BACKGROUND OF THE INVENTION

Coated bodies for use in the cutting of metal are well-known. Typically,the bodies are made of a cemented carbide, cermet or ceramic and thecoatings are one or more of a Group VIB metal carbide, nitride, oxide ormixtures thereof. For example, bodies of a cemented carbide coated withlayers of TiC, Al₂O₃ and TiN are widely used. There are many variationsin layer composition and thickness. The layers are applied by variousmethods such as CVD (chemical vapor deposition), both conducted atnormal temperatures of from about 900 to 1250° C. and medium temperaturechemical vapor deposition (MTCVD) conducted at temperatures of fromabout 700 to 900° C., and PVD (physical vapor deposition).

CVD TiC coatings are usually composed of equiaxed grains with the grainsize being from about 0.5 to 1.0 microns. CVD TiN as well as MTCVDTi(C,N) coatings are composed of columnar grains with the length of thegrains approaching the coating layer thickness. The morphology of CVDcoatings can be slightly modified by process adjustments. The MTCVDcoatings are, however, very difficult to modify by conventional processadjustments.

The hardness of polycrystalline materials in general (including coatinglayers as well) obey the Hall-Petch equation: H=H°+C/d where H is thehardness of a polycrystalline material, H° is the hardness of a singlecrystal, C is a material constant and d is the grain size. As may beseen from this equation, the hardness of a material can be increased bydecreasing the grain size. Nonetheless, conventional CVD and MTCVDcoatings have grain sizes of at least 0.5 microns and above. MTCVDcoatings are particularly characterized by the presence of largecolumnar grains with the length of the crystals approaching thethickness of the coating layer.

The use of a dopant such as a tetravalent titanium, hafnium and/orzirconium compound in the formation of an Al₂O₃ layer to promote theformation of a particular phase is shown in U.S. Reissue Pat. No.31,526. Also, the use of a dopant selected from the group consisting ofsulfur, selenium, tellerium, phosphorous, arsenic, antimony, bismuth andmixtures thereof to increase the growth rate of Al₂O₃ applied by CVD aswell as to promote even layers of the coating is disclosed in U.S. Pat.No. 4,619,886.

CO₂ has been used as part of the coating process as well. In particular,it has been used in oxidizing processes where it reacts with H₂ to formH₂O, the oxidizing gas. See, for example, U.S. Pat. No. 5,827,570.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to avoid or alleviate the problems ofthe prior art.

It is further an object of this invention to provide a coating layer ofsignificantly smaller grain size and concomitant hardness.

In one aspect of the invention there is provided a coated body having asa coating layer, a layer of Ti(C,N,O) having a grain size of 25 nm orless.

In another aspect of the invention there is provided a method of forminga coated body of Ti(C,N,O) comprising contacting a body with a gascontaining titanium halide, a nitrogen compound, a carbon compound, areducing agent and a dopant addition of CO and/or CO₂ sufficient to formthe Ti(C,N,O) in a size less than 25 mm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

It has now been found that the grain size of coatings applied by MTCVDcan be refined to much smaller grain size levels and to an equiaxedshape by adding small amounts of a dopant of CO or CO₂ or mixturesthereof, preferably CO, to the coating gas during the MTCVD process. Inorder to obtain a grain size of the resulting coating in the order of 25nm or less, preferably 10 nm or less, the amount of CO in the MTCVDgaseous mixture should be from about 5 to 10%, preferably from about 7to 9%, of the total gaseous mixture. When CO₂ is used, it should bepresent in an amount of from about 0.5 to 1.0%, preferably 0.4 to 0.6%of the total gaseous mixture. The CO and/or CO₂ dopant can be added atanytime throughout the reaction, continuously or in the interruptedmode. When CO₂ and/or CO/CO₂ mixtures are used, care should be takenwithin the skill of the artisan to avoid the formation of Magnelliphases.

While the dopant addition can be made to the reactant gas admixture usedfor various coating layers, it has found particular utility in theformation of a Ti(C,N,O) layer which would have been a Ti(C,N) layer inthe absence of the dopant. In the Ti(C,N,O) layer, the ratio ofconstituents generally has been as follows: O/Ti from 0.10 to 0.40,preferably 0.20 to 0.30, C/Ti from about 0.40 to 0.60, preferably 0.50to 0.60, and N/Ti from about 0.15 to 0.35, preferably 0.20 to 0.30.While a Ti (C,O,N) layer is preferred, the method of the presentinvention can be applied to form a Ti(C,O) layer which would have been aTiC layer in the absence of the dopant.

The nanocrystalline layer may be applied as the outermost layer or as aninner layer. As will be shown below, the nanocrystalline coatings areharder but exhibit at higher temperatures (at higher cutting speeds)grain boundary sliding leading to plastic deformation. Due to theextremely fine grain size of this coating, the surface smoothness isincreased and friction coefficient is reduced. Consequently,nanocrystalline coatings obviously are acting as frictionreducing/lubricating layers and should be consequently deposited atop ofthe existing coating structure. However, new coatings of MTCVD/CVD withalternating nanocrystalline layers (doping switched ON/OFF duringMTCVD/CVD process, nanolayered structures of MTCVD/nanocrystallinelayers are possible) should exhibit outstanding/new properties. Thenanocrystalline layers could also be used in combination with othercoating materials like alumina (kappa or alpha), or other oxides or TiNforming a nanolayered structure being composed of layers of MTCVD andnanograined coatings. Very thin nanocrystalline layers inserted in theMTCVD coatings can be used to control the grain size of the MTCVDcoating when a coating composed of mainly Ti(C,N) is preferred. Whenused as the outermost layer, the nanocrystalline layer may be appliedonto an Al₂O₃ layer, which itself can be applied onto one or more otherlayers such as, for example, TiC. The Al₂O₃ layer can be an alpha phase,a kappa phase or a mixture of alpha and kappa phase Al₂O₃. Thenanocrystalline layer may also be applied onto a TiN layer.

Similarly, when the nanocrystalline layer is applied as an inner layer,there may be other layers such as Al₂O₃, TiC, Ti(C,N), TiN or the likeapplied atop the nanocrystalline layer.

These various other inner and/or outer layers may be applied by CVD,MTCVD or PVD.

By equiaxed, it is meant that the grains have essentially the samedimension in all directions.

The invention is additionally illustrated in connection with thefollowing Examples which are to be considered as illustrative of thepresent invention. It should be understood, however, that the inventionis not limited to the specific details of the Examples.

EXAMPLE 1

In this case CO doping was applied. The following five experimentalcoatings (referred to as coatings 1, 2, 3, 4 and 5) were produced at apressure of 65 mbar according to the process data given in Table 1.

TABLE 1 CH₃CN H₂ (1/min) N₂ (1/min) (1/min TiCl₄ (1/min) CO % Coating 1balance 25 0.5 2 0 Coating 2 balance 25 0.5 2 3 Coating 3 balance 25 0.52 4 Coating 4 balance 25 0.5 2 6 Coating 5 balance 25 0.5 2 8

Grain Size

The coatings 1-5 were investigated using Transmission electronmicroscopy (TEM) in order to elucidate the effect of CO doping on thegrain size. It appeared clear that the microstructure of the MTCVDTi(C,N) coating being composed of large columnar crystals can bestrongly refined by CO doping. The structure became nanocrystalline at aCO doping level of about 8%.

X-ray Diffraction (XRD)

Coatings 1, 3, 4 and 5 were studied by using XRD. The grain refinementis clearly manifested as line broadening. In Table 2, data for linebroadening together with observed grain sizes are presented.

TABLE 2 FWHM* Line broadening Grain Size CO % (2φ°) (B_(n)/B₀) GrainShape (nm) 0.0 0.150 (B₀₎ 1.0  columnar 250** 4.0 0.209 1.39 columnar150** 6.0 0.330 2.20 columnar/equiaxed  50*** 8.0 0.359 (B_(n)) 2.39equiaxed  10   *Full Width at Half Maximum - measured fromK_(α2)-stripped Gaussian profiles of the 220 reflection of a singleMTCVD Ti(C,N) coating. **Average breadth of the columnar grains. Notethat the length can typically be of the order of the coating thickness.***Mixture of columnar and equiaxed grains. No columnar grainsapproaching the coating thickness could not be found. Full Width at HalfMaximum of the reference is B₀ Full Width at Half Maximum of thereference of the nanograined coating is B_(n) (n = 4.0, 6.0, 8.0) Linebroadening is B_(n)/B₀.

Line broadening is defined as absolute values (°2θ) and as relativevalues. The line broadening should be from 0.30 to 0.60 °2θ, preferablyfrom 0.33 to 0.4 °2θ, and from 2.0 to 4.0, preferably from 2.2 to 2.7(relative value, reference MTCVD Ti(C,N)).

The two definitions for line broadening are in this case characterizedby the fact that the coating is under slight tensile stresses. The linebroadening is consequently solely due to grain size alone and is not dueto compressive stresses together with a small grain size as is the casein PVD coatings.

Hardness

Hardness of the coatings 1, 3, 4 and 5 were measured by usingnano-indentation technique. The results are presented in Table 3.

TABLE 3 CO % Hardness Coating 1 0   26 Coating 3 4.0 28 Coating 4 6.0 29Coating 5 8.0 34

Coating Chemistry (Incorporation of Oxygen)

The experiments have shown that by doping, considerable amounts ofoxygen can be incorporated into the coatings, Table 4. It is clear thatthe carbon content in the coating is in principle, unaffected by theincrease in CO doping. The nitrogen level decreases, while there is adrastic increase in the oxygen content. However, titanium oxides(Magnelli phases) were not found. The stoichiometry of the coatinglayers increases from 0.88 to 1.03.

TABLE 4 Composition Coating 1 Coating 3 Coating 4 Coating 5 C/Ti 0.550.54 0.54 0.54 N/Ti 0.33 0.28 0.24 0.21 O/Ti 0.00 0.04 0.18 0.28 (C +N + O)/Ti 0.88 0.86 0.96 1.03

Friction

The friction coefficients between steel (SS1672) and the experimentalcoatings were measured using a pin-on disk method. Reduced frictioncould be confirmed, Table 5.

TABLE 5 CO % Friction Coefficient Coating 1 0   0.45 Coating 3 4.0 0.45Coating 4 6.0 0.41 Coating 5 8.0 0.32

Single layers of coatings 1, 3, 4 and 5 were deposited on turning(SNUN120408) and milling SEKN1203 AFN) inserts. All the coating layershad the same thickness of 6 μm. As is clear from Table 6, thenanocrystalline (coating 5) single layers exhibited good wear propertiesat lower cutting speed in turning. At the highest cutting speed, thecoating failed due to plastic deformation, however, with clearly reducedcrater, flank wear and chipping as compared to the non-doped coatinglayer (coating 1). In milling (in this case at medium speed) thenanocrystalline coating (coating 5) exhibited a clearly increased lifetime and enhanced edge strength (chipping resistance), Table 7.

From the results obtained, it is clear that a combination ofnanocrystalline coating layer atop a layer of Ti(C,N), TiN, TiC, Al₂O₃or a combination of these, obviously results in enhanced wearproperties, especially when turning at higher speed and chippingresistance in milling are concerned.

TABLE 6 Turning Stainless Steel (SS 2333) Life time (min) at 185 m/minLife time (min) at 250 m/min Coating 1 22 16 Coating 3 22 17 Coating 425 14 Coating 5 31  9 Lifetime criterion: surface finish or flank wearFeed: 0.4 mm/tooth Depth of cut: 2.5 mm

TABLE 7 Milling (SS 2333) Cutting Length (mm) Chipping % Coating 1 340012  Coating 3 3350 9 Coating 4 3800 9 Coating 5 4200 4 Cutting speed:200 m/min Feed: 0.2 mm/tooth Depth of cut: 2.5 mm

EXAMPLE 2

The nanograined coating layer was applied atop a MTCVD Ti(C,N) layer andatop at Ti(C,N)—Al₂O₃ multicoating layer. The coating combinations werethe following, Table 8. The coatings 1-4 were deposited on turning(SNUN120408) and milling (SEKN1203 AFN) inserts.

TABLE 8 Ti(C,N) Al₂O₃ Equiaxed Nano Grain Size Coating 1 6 μm — — —Coating 2 4 μm — 2 10 nm Coating 3 4 μm 4 — — Coating 4 4 μm 4 2 10 nm

TABLE 9 Carbon Steel, SS 1672 Life time (min) at 185 m/min Life time(min) at 250 m/min Coating 1 25 16 Coating 2 28 18 Lifetime criterion:Iso 3685

TABLE 10 Stainless Steel, SS 2333 Life time (min) at 220 m/min EdgeChipping (%)** Coating 3 12* 22 Coating 4 19   8 *Lifetime criterion ISO3685 **After 4 min of turning

TABLE 11 Cast Iron, SS 1672 Life time (min) at 220 m/min Edge Chipping(%)** Coating 3 15* 16 Coating 4 16  11 *Lifetime criterion ISO 3685**After 4 min of turning

TABLE 12 Face Milling (SS2377) Cutting Length (mm) Chipping %** Coating1  3400* 20 Coating 2 3350 15 Coating 3 3800 32 Coating 4 4200 16Cutting Speed: 80 m/min Feed: 0.6 mm/tooth Dept of cut: 6 mm Wet-milling*Lifetime criterion: surface finish **After 1800 mm

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A method of forming a coated body of Ti(C,N,O)comprising contacting a body with a gas mixture containing titaniumhalide, a nitrogen compound, a carbon compound, a reducing agent and adopant addition of CO and/or CO₂ sufficient to form the Ti(C,N,O) in agrain size of 25 nm or less, wherein the Ti(C,N,O) has a ratio of O/Tiof from 0.10 to 0.40, a ratio of C/Ti of from 0.40 to 0.60 and a ratioof N/Ti of from 0.15 to 0.35.
 2. The method of claim 1, wherein thetitanium halide is titanium tetrachloride.
 3. The method of claim 2wherein the nitrogen and carbon are supplied by the same compound. 4.The method of claim 3 wherein the nitrogen and carbon compound is CH₃CN.5. The method of claim 1 wherein the dopant addition is CO.
 6. Themethod of claim 5, wherein CO is present in an amount of from 5 to 10%of the total gas mixture.
 7. The method of claim 6, wherein CO ispresent in an amount of from 7 to 9% of the total of the gaseousmixture.
 8. The method of claim 1 wherein the dopant addition is CO₂. 9.The method of claim 8 wherein the CO₂ is present in an amount of from0.5 to 1.0% of the total gaseous mixture.
 10. The method of claim 9,wherein CO₂ is present in an amount of from 0.4 to 0.6% of the total ofthe gas mixture.
 11. The method of claim 1 wherein contacting the bodywith the gas mixture is performed at a temperature of from 700° to 900°C.
 12. The method of claim 1, wherein the grain size is 10 nm or less.13. The method of claim 1, wherein the ratio of O/Ti is from 0.20 to0.30, the ratio of C/Ti is from 0.50 to 0.60 and the ratio of N/Ti offrom 0.20 to 0.30.
 14. The method of claim 1, wherein the dopantaddition is supplied to the gas mixture intermittently.